The present invention relates in general to charge amplifier circuitry for use with capacitive transducers arranged for example in a towed hydrophone array as in a marine seismic streamer or an anti-submarine warfare streamer or the like, and more particularly to a differential charge amplifier for use in marine seismic streamer or anti-submarine warfare streamer applications involving towed hydrophone arrays for passive underwater detection, wherein the hydrophones drive long twisted pairs of leads or conductors for transmission through the streamer to shipboard signal processing stations.
Heretofore, a number of marine seismic detection cables or streamers, the terms being used interchangeably, have been devised for making seismic surveys of terrestrial sub-surface structures disposed beneath sea water. In general, the marine seismic streamers have included a lead-in cable and a long series of serially connected active streamer sections each usually formed of an oil filled plastic tube surrounding an array of hydrophones, strain cables, structural spacers, transformers, and mechanical and electrical leads or connectors, such as, for example the marine seismic streamer sections of the type disclosed in U.S. Pat. No. 2,465,696 issued Mar. 29, 1949 to Leroy C. Paslay or U.S. Pat. No. 3,371,739 issued Mar. 5, 1968 to Raymond H. Pearson. Inactive streamer sections are also often interspersed in the string of serially connected so called "active" sections having the sound pressure responsive hydrophones. Such marine seismic streamers or cables may in many cases be a mile or more in length, a typical streamer system being about 7,000 with each section being typical a hundred feet or more in length. During seismic survey or prospecting operations, such streamers are towed by the seismic survey vessel at a selected depth below the surface of the sea, by any of several conventional means for maintaining the seismic cable at the desired underwater depth. Typically, the cables may be provided with a plurality of weights at spaced intervals to make them negatively buoyant and flotation means or ring boom means may be associated with the cable to assist in maintaining it at the desired depth. Alternatively, regulation of the buoyancy of the streamer may be achieved by introducing into the streamer or withdrawing from it a fluid which will vary the buoyancy of the streamer, or paravane type structures may be used having adjustable diving planes which maintain the streamer at the appropriate depth.
Similarly, long towed hydrophone arrays have been employed as underwater listening devices for detecting submarines or underwater vessels used in warfare, such towed hydrophone arrays being employed as passive underwater detection systems in what are referred to as ASW streamers.
In towed underwater hydrophone arrays of either of the types described above, piezoelectric ceramic capacitance transducers are customarily used as hydrophones to respond to the underwater sound pressure waves or phenomena to be detected by the hydrophones and convert such phenomena to electrical information. Such transducers produce an output voltage that is proportional to the applied acoustic pressure and present day streamer applications typically employ up to one hundred groups of hydrophones towed in a single streamer, spaced from 500 to 10,000 feet behind a marine geophysical boat. In order to transmit the hydrophone output signals or output voltages to the signal processing equipment on the geophysical boat or the towing vessel, it has been the practice to connect a transformer to each hydrophone group as means of overcoming the change of signal loss over such a long line with variations in the cable length. In such a streamer systems involving transformers connected to the hydrophone groups, the transformer basically lowered the source impedance allowing the hydrophone group to drive a long twisted pair of cables for coupling the output signals from the hydrophone group to the processing equipment on the towing vessel. Such a system, however, has well recognized disadvantages, in that the output voltage from the hydrophone group is substantially reduced, in some cases by as much as 10 to 1. While a typical multidyne hydrophone may have an output of about 56 uv/ubar, at the transformer output in such a system this may be reduced to about 5-10 uv/ubar depending on the number of phones and transformer design, thus resulting in a serious loss of sensitivity of the hydrophone system and rendering it extremely difficult to obtain adequate signal-to-noise ratios where long lines are required to conduct the signals from a streamer to the towing vessel.
Charge amplifiers have been previously employed for certain types of transducers, such as capacitance microphones and some types of accelerometers, where the transducers operate on the principle of conversion of some mechanical, thermal, chemical, etc. phenomenon to an equivalent electrical charge. To complete the charge amplifier circuit, an Operational Amplifier is connected with its inverting input or negative input connected to the transducer, and with a feedback circuit including a capacitor and a resistor in parallel connected between the output and the negative input of the Operational Amplifier. It has been found that such a circuit has the desirable property of being virtually independent of shunt capacitance across the input of the Operational Amplifier. When using negative feedback, the Operational Amplifier will work in such a manner as to keep the two inputs at the same potential, and since the + input is connected to ground, the - input is also kept at ground potential by the negative feedback. Therefore, adding shunt capacitance across the input has no effect because no voltage is developed across the shunt capacitance.
Such a conventional charge amplifier, if employed in a towed hydrophone array application of the type described, wherein piezoelectric ceramic capacitance transducers are used as hydrophones, would seem to offer hope of eliminating the use of transformers associated with the hydrophone groups and the resultant loss of sensitivity, because of the independence of the charge amplifier of shunt capacitance across the input. However, if a charge amplifier were connected to the hydrophone transducers in conventional fashion, a number of problems are presented rendering the conventional charge amplifier apparently unsuitable for such an application, including particularly the property that, because of the unbalanced input present in the basic conventional operational charge amplifier circuit, the circuit has no common mode signal rejection capability.
An object of the present invention is the provision of a differential charge amplifier circuit to be connected to a group of hydrophones in a towed hydrophone array employing long twisted pairs of cable leads or conductors for conducting the electrical signals to the towing vessel, which circuit allows the array of hydrophones to drive the long twisted pairs of leads without the need for a transformer and the resulting loss of sensitivity.
Another object of the present invention is the provision of the novel differential charge amplifier circuit for use with hydrophones of a towed array of hydrophones of the piezoelectric ceramic capacitance transducer type, arranged for example in a towed underwater marine streamer for seismic or ASW applications and the like, which provides a balanced input to the differential amplifier providing common mode signal rejection capability, and which provides good frequency response characteristics over the band of frequencies appropriate for seismic prospecting and ASW detection applications.
Another object of the present invention is the provision of a differential charge amplifier circuit as described in the immediately preceding paragraph, which minimizes changes in frequency response or phase shift resulting from electrical leakage in the streamer sections where the hydrophones are located or resulting from sea water intrusion and the like, minimizing the need for removal and replacement of the hydrophone sections.
Other objects, advantages and capabilities of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings illustrating a preferred embodiment of the invention.